Image scanners are widely used for scanning documents. In order to enhance the image quality, the image data of the scanned documents are usually subject to image compensation and then transmitted to computers or other data processing devices.
As known, uneven luminance of the lamp tube is likely to occur because the luminance at both ends of the lamp tube is different from the luminance at the middle portion thereof. In addition, during the process of assembling the image scanner, fabrication errors may be generated from some components such as the charge-coupled device (COD). Typically, a shading compensation method was used to compensate the image distortion due to the uneven luminance of the lamp tube and the fabrication errors.
For performing the shading compensation, the pixel data of the scanned document is multiplied by the X-axis compensation gain and the offset value is also incorporated. Depending on the positions of the pixel data for the same scan line, the X-axis compensation gain is varied. That is to say, the shading compensation is used to compensate the errors generated in the horizontal direction.
In order to shorten the warm-up time period of the image scanner, it is necessary to perform the Y-axis compensation to compensate the errors generated in the vertical direction in some circumstances.
A compensation method for compensating the errors generated in the vertical direction was disclosed in for example Taiwanese Patent No. 1243591, entitled “Compensation method of digital image data”, which was filed by the same assignee. As disclosed in the patent, the luminance of the lamp tube may approach stable after the image scanner has been turned on for a certain time period. This time period is referred to the warm-up time period. The scanning operation should be performed after the luminance of the lamp tube reaches the stable state. In order to perform the scanning operation with no warm-up time period, this compensation method uses software to compensate the image data in the vertical direction. Accordingly, the pixel data for the same scan lines are multiplied by the identical compensation gain. Whereas, different compensation gains are used to compensate the pixel data for different scan lines.
For a purpose of performing the no warm-up scanning operation, it is necessary to perform the Y-axis compensation. Furthermore, the present inventors found that Y-axis compensation was also inevitable when the image scanner has been used for a long term.
Referring to FIG. 1, a plot of the luminance variation of a lamp tube is illustrated. In the initial stage from start of the image scanner, the luminance of the lamp tube is slowly increased. Later, after the image scanner has been turned on for a certain time period, e.g. t1 seconds, the luminance of the lamp tube approaches stable. Unfortunately, when the image scanner has been used for a long term, e.g. t2 seconds, the luminance of the lamp tube falls down and the scanning performance will be impaired. Under this circumstance, the Y-axis compensation should be use to compensate the image data.
In a case that the image scanner has an automatic paper feeder to successively scan 10˜50 paper sheets at a time, the total time of scanning plural paper sheets is much more than that of scanning one paper sheet. In other words, unlike the scanning operation of scanning one paper sheet, it is necessary perform the Y-axis compensation when plural paper sheets are scanned.
Although the compensation method disclosed in Taiwanese Patent No. 243591 performs the Y-axis compensation by using related software, it still has some drawbacks. For example, the software should be installed in a personal computer before the scanning operation, and some users may have problem in realizing how to install the software. Furthermore, a multifunction peripheral having multiple functions in one structural unit, for example the functions of a printer, a scanner, and optionally a fax machine and/or a copy machine, may be individually operated without communication with the computer. Under this circumstance, using the computer software is not applicable to the multifunction peripheral.
In views of the above-described disadvantages of the prior art, the applicant keeps on carving unflaggingly to develop a controller of a scanning apparatus according to the present invention through wholehearted experience and research.